1. Field of the Invention
This invention relates generally to aircraft flight control instruments, and to the generation and display of commands to allow the flight crew of an aircraft to exit a windshear encounter in an optimal manner. More particularly, it provides a variable speed command derived from the magnitude and duration of the windshear condition, operative between predetermined maximum and minimum safe speed limits.
2. Description of the Prior Art
The phenomenon of windshear can pose a serious threat to the safety of aircraft and in fact has been directly responsible for several serious aircraft accidents. Windshear, either of itself or as a result of attempts by the human pilot to restore the aircraft to its normal flight path, can cause the aircraft to stall or crash. Windshear can be defined as the time rate of change of wind relative to the aircraft, whose effect on the aircraft is to cause large speed or altitude deviations from normal flight. This definition requires that atmospheric turbulence, maneuvering into and out of steady winds, and penetration of a constant wind boundary layer near the ground all be considered windshears since they represent boundary conditions relative to the aircraft. However, usually the magnitude and duration preclude these shears from posing a threat to the aircraft. While the definition requires a rate of change in wind, a constant vertical wind is also considered to be a shear throughout the industry. While not accurately a shear, the effect on the aircraft is identical to a longitudinal wind changing at an equivalent rate.
In the prior art windshear warning systems the detection and guidance provided during a windshear encounter would cause the aircraft to fly at some fixed speed, usually slightly greater than stall speed. The speed commanded was usually a speed known as stick shaker speed, approximately five percent greater than stall speed, and is the speed where artificial means are used to vibrate the control column or stick to warn the human pilot of impending stall. Stick shaker speed is generally considered to be the minimum speed for safe flight, and varies with the angle of attack of the aircraft and flap position. By reducing his forward speed, the pilot is able to gain altitude rate.
As many commercial transport aircraft, general aviation aircraft, and military aircraft are equipped with a Flight Director System whereby pitch command signals may be displayed to the human pilot, the speed command for a windshear encounter is usually presented as a displacement of the pitch command bar. When the human pilot maneuvers the aircraft in such a manner as to reduce the displacement to null, the speed of the aircraft will be at the commanded speed, since the speed is a function of pitch for constant engine thrust.
A shortcoming of the prior art is that the commanded fixed speed may result in the aircraft flying at the minimum "safe" speed when the magnitude and duration of the windshear do not in fact require such a maneuver. Flying at the minimum safe speed results in a degradation of the speed margin of the aircraft and hence reduces the margin of error allowable to the human pilot in controlling the aircraft. Consequently, prior art systems could in fact create a potentially dangerous situation wherein the speed margin of the aircraft was diminished substantially, even though the magnitude and duration of the windshear did not warrant it.
Conversely, prior art systems which utilize a commanded fixed speed significantly greater than stick shaker speed do not command the aircraft to a diminished speed adequate to cope with the windshear condition for windshears whose magnitude and duration warrant the stick shaker speed command.
Prior art systems, in summary, failed to recognize that the important command parameter is not a fixed air speed command but is in fact a variable air speed command derived from the rate of change of air speed due to the windshear encounter. The present invention determines the correct air speed rate command based on both the magnitude and duration of the windshear condition and hence overcomes the shortcomings of the prior art in that its command causes the aircraft to fly at the speed appropriate to the magnitude and duration of the windshear encountered.